Surface treatment of previously graphitized carbonaceous fibers

ABSTRACT

MODIFIED IN ACCORDANCE WITH THE PRESENT PROCESS IN A RESINOUS MATRIX MATERIAL.   AN IMPROVED PROCESS IS PROVIDED FOR MODIFYING THE SURFACE CHARACTERISTICS OF A PREDOMINANTLY GRAPHITIC CARBONACEOUS FIBROUS MATERIAL THEREBY TO FACILITATE ENHANCED ADHESION BETWEEN THE FIBROUS MATERIAL AND A MATRIX MATERIAL. THE FIBROUS MATERIAL IS CONTINUOUSLY PASSED FOR AN EXTEREMELY BRIEF RESIDENCE TIME TRANSVERSELY THROUGH THE TIP OF AN INERT GAS THERMAL PLASMA PRESENT IN AN OPEN ATMOSPHERE. COMPOSITED ARTICLES OF ENHANCED INTERLAMINAR SHEAR STRENGTH MAY BE FORMED BY INCORATING THE FIBERS

K. c. Hou 3,767,774 SURFACE TREATMENT OF PREVIOUSLY GRAPHITIZED CARBONACEOUS 2 Sheets-Sheet 1 Oct. 23, 19.73

Filed Aug. 12, 1971 Oct. 23, 1973 K. c. Hou

SURFACE TREATMENT OF PREVIOUSLY GRAPHITIZED CARBONACEOUS 2 Sheets-Sheet 2 Filed Aug. 12 1971 NvENToR C. HOU

KENNETH United States Patent U.S. Cl. 423-447 10 Claims ABSTRACT F: THE Dlscnosurtn An improved process is provided for modifying the surface characteristics of a predominantly g'raphitic carbonaceous fibrous material thereby to facilitate enhanced adhesion'between the fibrous material anda matrixmaterial. The fibrous material is continuously passed for an exteremely brief residence time transversely' .through the tip of an inert gas thermal plasma present in an openV atmosphere. Composite articles of enhanced interlaminar shear strength may be formed by incorporating 'the fibers modified in accordance'-with the presentv process. inv a resinous matrix material'.

BACKGROUND OF THE INVENTION In the search for-'high perforrnance materials, considerable interest has been focused upon graphiticcarbon'fibers Graphite fibers are defined herein fibers'which consist essentially 'of carbon `and have' afpredominantl Xfr'ay diffraction pattern characteristic' ofv graphite. Amorphous carbon fibers, on the other hand, are defined as fibers in which the bulk of the fiber-weightfcan be attributedv to carbon and which exhibit ain .essentially amorphous X-ray diffraction pattern. Graphite fibers generally have a higher Young's modulus than do amorphous carbon fibers and in addition are more' highlyv electrically and thermally conductive.- v

Industrial high lperformance materials' of the future are projected to make substantial utilization of fiber reinforced composites, and graphiticcarbon' fibers theoretically have among the best properties of any ber'for use as high strengthv reinforcement. Among tliese'desi'rable properties are corrosion andi hightemperature resistance, low density, high tensile strength, and high4 modulus. Graphite is one of the very few known materials whose tensile strength increases with temperature. Uses for graphitic carbon fiber reinforced composites include aerospace structural components, roeketmotor casings, deepsubmergence vessels y'and ablative materials for heat shields on. re-entry vehicles.l' t

In the prior art numerous materials have been proposed for use as possible-matrices in--whichgraphitic carbonv fibers may be incorporated tolprovide'reinforceme'nt and produce a composite article. The matrix material which is selected is commonly a thermosett'ing'resinous material and is commonlyv selected because of its ability to also withstand highly elevated temperatures.

While it has been possible in the past to provide graphitic carbon fibers of highly desirable strength and modulus characteristics, difficulties have arisen when one attempts to gain the full advantages of such properties in the resulting carbon fiber reinforced composite article. Such inability to capitalize upon the superior single fila'- ment properties of the reinforcing fiber has been traced to inadequateadhesion between the fiber and the matrix inthe resulting composite article. t

Various techniques have beenproposed in the past for modifying the fiber propertiesofa previously formed carbon fiber in order to make possible improved adhesion when present in a composite article. See, for-instance,

US. Pat. No. 3,476,703 and British Pat. No. 1,180,441

3,767,774' Patented Qct. '23., 1 973 to INicholasl..Wadsworth'and William Watt wherein it is taught to heat a,carbon fiber norrgnallyvwithinlhev range of 3502.` to 850 C. (eg. 500 to 600," C.)v. in anexidizing atmosphere such as air for a'n appreciable period of time. 0,ther. atmospheres,eomtemplated for use in the process include an oxygen rich atmosphere, pure oxygen, or an atmosphere-containing an oxide of nitrogen from which free oxygen becomes available such as nitrous oxide and nitrogen-dioxide. Improved carbon liber surface modification processes are disclosed in commonly assigned U.S. SerkNos. 65,454-and65,456, liledvAug. 20, 1970; and U.S.v Ser. No. 99,169, tiled Dec'. 17,'- 1970.

- It isan object of the invention top'rovide a :continuous process for'rapidly. and 'efficiently-modifying the surface characteristics of graphitic carbon fibers.` 1

It is an object of theinvention to provide ain improved process for improving the ability of graphi-tc carbon bers to bond to a resinous matrix material. y

It isan object of the invention to provide a process for modifying thefsurface characteristics of graphitic carbon bers which eliminates the need.` for extendedtreatment periods` n. 1 A e It is anotherobiect of the invention tol provideeom-` posite articlesrelnforced with vgraphitic, carbonVv fibers exhibiting improved inter-'laminar shear strength.

It is a further object of the invention to provide an' improved process for improving the abilitymt' graphitic carbon fibers to bond to a'resinous matrix material in the absence of any substantial diminutionof sing-le lament tensile properties'.l g These and other objects, as well asthe--scope,`nature, and utilization of" the'. invention will be apparent., from the following detailed description and appended.claims.

` SUMMARY QF THEQINYENTION' .j It has been found that a process for-the surface treatment of apredominantly graphitic carbonaceous fibrous material comprises: l (a) electrically inducing heating of a stream of'inert gas i present within a laterally enclosed zone to athernial plasma state 'of at least about one atmosphere pressure wherein from about-5 toabout 50 percent of the ga's is in an --ionized state, and wherein the tip of the thercontinuous lengthfof-a previouslyl graphitized'carbonaw ceou's fibrousfmaterial containing at least about 95 percent carbon by weight and exhibiting a predominantly graphiticiX-ray diffraction'pattern transversely through i' the 'tipofthe resultingithem'tal plasma present in the open atmosphere for aresidence' time of aboutOi-t to bonaceous tibrousmateria-l'- isheated to a maximum ternperature'of about 150G-to 2200 C. and the surface characteristics .thereofare laeneficiall-yV modified and renderedv capable ofenhancedl adhesion with a matrix material.

DESCRIPTION oF DRAWINGS process ofthe present invention.

' FIG. 2 isa photograph made with the aid. of a scanning electron microscope ata magnification of 6000 of a portiowcf agraphitic carbon filament vwhich has not undergone surface modification. j FIG. 3 is a1 photograph made with the aid of a scanning electron microscope at a magnification of 6000 of a portion a similar graphitic carbon filament has I filament tensile properties thereof substantially diminished.'

DESCRIPTION OF PREFERRED EMBODIMENTS The starting material y i Y I The carbonaceous fibers which are modified in accordance with the process of the presentinvention contain at least about 95 percent carbon by weightand 'exhibit a predominantly graphitic X-ray diffraction pattern. In a preferred embodimentof the process the vfgraphitized carbonaceous fibers which undergo surface treatment contain at least about 99-percent carbon by weight.

The graphitized carbonaceous rfibrous materials are provided in a continuous length in any one of-a variety of physical configurations provided substantial access to the fiber surface is possible during the surface modification treatment'described hereafter. For'- instance, the fibrous materials may assume'the configuration of a-.continuous length of a multifilament yarn,-tape, tow, strand, cable, or similar fibrous assemblage. In a preferred embodiment of the process the fibrous material lis one or more continuous multifilament yarn. When a plurality of mul` tifilament '-yarns vare surface treated simultaneously (as described hereafter),I they may be continuously passed through the treatment zone while in parallel and in the form ofla fiat ribbon.-

The previously graphitized carbonaceousfibrous material which is treated in the present processl optionally-may be' provided with a twist-which tends to improve the handling characteristics. For instance, la twist of about 0.1 to 5 t.p.i.,and preferably about 0.3 to 1.0 t.p.i., may be imparted to a multilament yarn. Also, a false twist may be used instead of or in addition to a real twist. Alternatively,one may select continuous bundles ofbrous material which possess essentially no twist.

The graphitized carbonanous fibers which serve as the starting material in the present process may be formed in accordance with a variety of techniques as will be apparent to those skilled in the art. For instance, organic polymeric fibrous materials which are capable of undergoing thermal stabilization may be initially stabilized by treatment in an appropriate atmosphere at a moderate temperature (e.g. 200 to 400 C.), and subsequently heated in an inert atmosphere at a more highly elevated temperature until a carbonized and graphitized fibrous material is formed. For4 instance, the thermally stabilized material may be carbonized by heating in an inert at mosphere at a temperature of about 900 to 1000" C. and subsequently heated to a maximum temperature of 2000 to 3100 C.- (preferably 2400 to 3'100" C.) in an inert atmosphere for a sufficient residence time to produce substantial amounts of graphitic carbon.

The exact temperature and atmosphere utilized during the initial stabilizationvof an organic polymeric fibrous material commonly vary with the composition of the precursor as will be apparent to those skilled in the art. During the carbonization reaction elements present in the fibrous material other than carbon (e.g. oxygen and hydrogen) are substantially expelled. Suitable organic polymeric fibrous materials from which the graphitized carbonaceous fibrous materials may be derived include an acrylic polymer, a cellulosic polymer, a polyamide, a polybenzimidazole, polyvinyl alcohol, etc. As discussed hereafter, acrylic polymericmaterials areparticularly suited for use as precursors in the formation 'of graphitizcd carbonaceous fibrous materials. Illustrative examples of suitable cellulosic materials include the natural and regenerated forms of cellulose, e.g. rayon. Illustrative examples of suitable polyamide materials include the aromatic polyamides, such as nylon 6T, which is formed by the condensation of hexamethyienediamine and terephthalic acid. A n illustrative example of a suitable polybenzimidazole is poly- 2,2' m phcnylene 5,5 bil benzimidazole.

A fibrous acrylic polymeric material prior-to stabilization may be formed primarily of recurring acrylonitrle units. For instance, the acrylic polymer should contain not less than about mol percent of recurring acrylonitrile units with not more than l5 mol percent of a lmonovinyl compound which is copolymerizable with acrylonitrle 4such as styrene, methyl acrylate, methyl methacrylate, vinyl acetate, vinyl chloride, vinylidene chloride, vinyl pyridine, and the like,.o r a plurality of such monovinyl compounds.

During the formation of a preferred graphitized carbonaceous fibrous material for use in the present process multifilament bundlesrof an acrylic fibrous material may be initially stabilized in an oxygen-containing atmosphere (i.e. preoxidized) on a continuous basis is accordance with the teachings of U.S. Ser. No. 749,957, filed Aug. 5,v

` The stabilized acrylic fibrous .material which is preoxidized in an oxygen-containing atmosphere is black in appearance, contains a bound oxygen content of at least about 7 percent by weight as determined by the Unter-- zaucher analysis, retains its original fibrous configuration essentially intact, and is non-burning when .subjected to an ordinary match flame.

In preferred techniques for forming the starting material for the present process a stabilized acrylic fibrous material is carbonized and gmphitized while passing through a temperature gradient present in a heating zone in accordance with the procedures described in commonly assigned U.S. Ser. Nos. 777,275, filed Nov. 20, 1968 of Charles M. Clarke, now abandoned; 17,780. filed Mar. 9, 1970 of Charles M. Clarke, Michael I Ram, and John P. Riggs, now U.S. Pat. No. 3,677,705; and 17,832, filed Mar. 9, 1970 of Charles M. Clarke, Michael I. Ram, and Arnold J. Rosenthal. Each of these disclosures is herein incorporated by reference.v y

In accordance with a particularly preferred carbonization and graphitization technique a continuous length of stabilized acrylic fibrous material whichis non-burning when subjected to an ordinary match Vflamef and derived from an acrylic fibrous material selected from the oroup consisting of an acrylonitrle homopolymer and acrylonitrile copolymers which contain at least about 85 mol percent of acrylonitrle units and up to about 15 mol-percent of onefor more monovinyl units copolymerized therewith is converted to a graphitic fibrous material while preserving the original fibrous configuration 'essentially intact while passing through a carbonization/graphitization heating zone containing an inert gaseous atmosphere and a temperature gradient .in which the fibrous material is raised within a period of about 2t) to about 300 seconds from about 800 C. to a temperature of about l600 C. t0 form a continuous length of carbonized fibrous material, and in which the carbonized fibrous material is subsequently raised from about 1600 C. to a maximum temperature of at least about 2400 C. within a period of about 3 to 300 seconds where it is maintained for about l0 seconds to about 200 seconds to form a continuous length of graph'itic fibrous material.

The equipment utilized Ato produce the heating zone used to produce the graphitized carbonaceous starting material. may be varied as will be apparent to those skilled l in the art. It is essential; that the apparatus selected be capable ofproducingthe required temperature while excluding the presence' of an oxidizing atmosphere.

In a preferred technique the continuous length of fibrous,l material undergoing carbonization and graphitization is. heated by use-vofan induction furnace. In such a procedure the fibrous vmaterial may be passed in the direction otitevv length through a hollow graphite tube or other susceptor which is situated withinthe windings of an induction coil. By varying the length of the graphite tube,

vided at a temperature of aboutI 4,000 to 12,QQO". K; If,

l field is oorioorltratod, at the tip. of 'tho oorldllotor and the.

apparent to those `skilled in the art. The fibrous material v because of itst small mass yand relatively large surface area instantaneously assumes substantially the same tem- 1 Poratllre as that. ofr the urbanization/graphitizatioll hooi ing z one through which it iis-continuously passed.

surfe-Qt treatment During the surface 'treatment process ofthe presentinvention, 'a stream of gas present ina laterally enlosod zone-is hesiod vla tleotr'ial induction of a thermal plasma state of atleast about one atmosphere pressure (es. l tol atmospheres, pressure) wherein from about 5 to about 50 'ent ofthe gas is in an ionized state, and Whill'ill lll? lill. LG.. the tail, of the thermal plasma extends beyond the coufinespof the laterally enclosed `zolle into .an @pen atmosphere. The percent of the gas in an ionized statev may be determined by spectroscopic measurements, as will b e apparent to those skilled in the art.

Preferred inert gases for use in the process are monoatomic, e .g. argon,'heliu 'm, neon, z enon and radon. ParticularlyProforredmonoatomio inert eases are arson and helium.' A` diatomio inert gas such as' nitrogen may alterspiral flow present the` gas stream accordingly creates a thermal pinch :effect or a vortex stabilization effect wherein thelhermal plasma state is positioned within the lcentral portion of the zone in the substantial absence of contact with the periphery of the laterally enclosed zone. The'periphery of the laterally enclosed zone `rnay therefore be formed from conventional materials,

such as aquartztube. A cooling medium may also cir- .culatewithin the vwall .of the zone.

The temperature of the gas throughout the thermal plasma commonly ranges from about 4,000 to 20,000 K. Accordingly, the thermal'plasma state is substantially different than conventional plasmas (e.g. glow discharges) which arecncountered in neon lights, etc., due to the ex- I tremely high temperatures in the former because ofthe higher pressure of the gas present and the highervdegree of ionization.

The tip or tail of the thermal plasma which extends into an open atmosphere commonly measures about 0.5 to 8 inches, `or more, in length and preferably about 2 to 4 inches in length. The tip of, the thermal plasma is comdesired. tho. oolllleilratiorloff tho till.. ofthe. llitrtllellllasma may be modified by the of a de liector. The length of the thermal plasma tioalso may. b, modiliotl by a'iillnins the liow rato, ottllo. inert-llas The thermal plasma utilized irl the present process is creatori by all tlscirodsle es ttshrliqllo- Ifrl assordance with this, technique the thermalgplasma Vstate is achieved by heatirlsA tbe" setto ioll'iratiolltomlioratllrt by means of oltotrisal. induction created Prsfer'ably by.' 'a sufrrounding coil carrying radio frequenoy (e.g. 0.5 kHz. to 2500 mHz., and preferably lV to, 30 mHz.)"cur-rent ofv 3 to 100 lwfl'he thermal plasma be initiated by temporarily insortirl a, tondllstor ruoli as. a metallic wire or graphite rod into the radio f rcquency field wherebythe breakdown potential of the "inert gas stream lowered. A tirlittA loro it established within til@ roaotor'wlltroin the irlort sas exists in the. this` .al-plasma stata Byaffliutins and maintaining tbs.- flow rato of the inert ses. tbc iiipiit of power. and tho rizo O f the Zoll@ wherein. tlo sas .triste the thermal plasma slate 'may be effectively controlled. The length-of the thermal plasma is alsoirlduenced to a substantial degree by the lonstb of- 'the surrounding coil. If desired. the. massellolielsi may bs .Stro's'tlislltd through the permanent positioning of s' illtta'llio sheet" within the magnetic, `field created bvftho raslis'frolqllorlsy power'.

Onto all irisrt ses lhtrlrlal rlasmastate' iscreatedvwith the tip thereof. which exhibits a temperature of about 4.000 to' 12,000"` K. extending ll'ltlv an. ononatrnosphero. the continuous length of previo graphitized carbo.- naceous fibrous material may be continuously passed'in the direction of its length transversely through the same for a residence time of about 0.4 to 3.5V seconds, and more preferably for a vresidence time of about l to 2 seconds. When the fibrous material is provided asa relatively more cornpact assemblage of a plurality o f'fibers, then longer residence times within the range indicated advantageously may be employed. Also. graphitic carbonfibrous materials having an extremely high hfung's modulus tend to require longer residence timesfWhile present within the vtip of the thermal plasma, the fibrous material is heated to a maximum tempfatlll's Qf about 1509 0.2200. C., and more preferably to a maximumtemperature of about 1600 to 2000 C. The maximum temperature imparted to the fibrous material may be varied depending upon the exact portion lof the plasma tip which is transversed. The portion ofthe tip closest to the laterally enclosed zone will tend to heat the f i us material to a higher maximum temperature. The temperature of bl'Qus material may be determined by optical grignoter ttohoitiuos' which are well known in the art. f

' It desired. the basic proton of tbs invention. may be ,th odllolion of o' millor quantity of an additiye, such as oxygen, into the inert gas Stream which is indllslivoly sonvertoilto a vthermal plasma state. Alternatively, inorganic powders, such as boron ni-f-v tride or boron carbide, can be'introduced into the inert gas stream, andV ultimately deposited upon the'surface of the fibrous material undergoing treatment.

The theory whereby the surface of a previously graphitized carbonaceous kfibrous material is modified in the presentr process is considered complex andfincapable of simple explanation. It is believed, however, that the resulting modification is induced by a combination of physo ical and chemicalinternctions between the ionized inert gas, molecular oxygen provided by the open atmosphere, and the fibrous "material,

The surface treatment of the present process is accomplished in the absence of any-substantial diminution of the single filament tensile properties of the fibrous material. More specifically, the single filament tensile properties (Le. tensile strength and Youngs modulus) are substan tially identical following the surface treatment to those properties exhibited prior to the surface treatment. A comparison of fibers of FIGS. 2 (control) and 3 (surface treated in accordance with the present process) indicates that the appearance of the fibrous material following the surface treatment is substantially unchanged, with only a very shallow pitting of the fiber surface being observable. Both FIGS. 2 and 3 show a finely textured surface on the lobes of the fiber; however, the troughs between the lobes of the fiber of FIG. 3 appear to be slightly rougher in texture. FIG. 4 indicates the appearance of a fiber which has undergone an excessive and undersirable surface treatment and is excessively pitted.

The surface modification imparted to the previously carbonaceous' fibrous material through the use of the present process has been found to exhibit an appreciable life which is not diminished to any substantial degree even after the passage of 30, or more days.

The surface treatment of the present processmakes possible improved adhesive bonding between the resulting fibrous material, and a resinous matrix material. Accordingly, graphite fiber reinforced composite materials which incorporate fibers treated as heretofore described exhibit enhanced shear strength, flexural strength, compressive strength, etc. The resinous matrix material employed in the formation of such composite materials is commonly a polar thermosetting resin suchas an epoxy,v a polyimide, a polyester, a phenolic, etc. The graphitized carbonaccous fibrous material is commonlyprovided ink such resulting composite materials in either an aligned or random fashion in a concentration of about 20 to 70 percent by volume.

The following examples are given as specific illustrations of the invention. It should be understood, however,

that the invention is not limited to the specific details setforth in the examples. Y.

In each example a continuous length of previously graphitized carbonaceous continuous multifilament yarn containing in' excess of 99 percentcarbon by weight was subjected to the surface treatment of the present invention utilizing the apparatus of FIG. l. l

The apparatus wherein the surface modification treat` ment was conductedvincludes a vertical quartz tube 1 having a length of'6 inches, an inner diameter of 1% inches, and a` wall thickness of M6 inch.4 A larger concentric quartz tube 2 having an internal diameter of 2% inches and a wall thickness of l/ip, inch is provided outside quartz tube 1 and forms a wall cavity 4 wherein a water cooling medium is continuously circulated. The cool'- ing medium enters inlet 6 and exits at outlet 8. The lower ends of quartz tubes 1 and 2 are mounted in steel base 10. Also mounted in steel base 10 is a relatively short section of cylindrical Pyrex glass tubing l2 having a length of 2 inches, an inner diameter of 1 inch and a wall thickness of ly inch which is provided with rperforations (not shown) to aid in the mixing of the inert gas. An annular space 14 is accordingly provided between the inner wall of quartz tube 1 and the outer wall of Pyrex glass tubing 12. Gas inlet tube 16 passes through base 10 and is directed longitudinally along the central region of the zone 20 defined by quartz tube 1. Gas inlet tube 22 also is situated within base 10 and communicates tangentially with annular space 14.

A hollow copper induction coil 24 having an outer diameter of A inch and a wall thickness of M52 inch is wound about and in contact with the outer wall of quartz tube 2. The continuous coil has 5 turns which are each separated by 3/s inch. Water is continuously circulated through hollow copper induction coil 24.

Argon gas is introduced through inlet tube 22 tangentially into annular space 14 at a rate of 20 EL3/hr. This gas accordingly passes spirally along the internal walls of Y zone 20 after passing through Pyrex glass tubing 12..The' pressure ofthe gas stream passing upwardly through zone 20 is slightly in excess of l atmosphere. Quartz tube `l. is open to the air atmosphere at 32.

A thermal plasma 40 is electrically induced within the zone 20 following 'the temporary insertion of a metallic wire (not shown) through opening '32 whereinabout 5 to 15 percent of the argon gasnisin an ionized state. Radio frequency power is applied tothe copper induction coil 24 via leads 42 and 44 and highfrequency-l power supply 46.y The power supply 46 consists of a Lepel Model No. T-IO-S-MC-E-S high frequency power unit capable of delivering up to a l0 kW signal at a frequency of up to 8 mHz. A 10 kv. peak-topeak A.C..signal having a frequency of S mHz. is applied. 1

The thermal plasma 40 has aV total length of about 6 inches, and a maximum width' of 1%' inches. Approximately 3 inches of the thermal plasma 40 extend beyond the open end 32 of vertical quartz tube 1 into the air atmosphere. The temperaturev of the gas within the thermaly plasma ranges from about 4,000 K. at the extreme tip to about 8,000 K. atqthe area surrounded by coil 24. The temperature of the gas-` within the tipportion of the thermal plasma present within the air atmosphere ranges from about 4,000 to 6,000 K.; A

The previously gr'aphitled carbonaceous yarn 50 is next continuously'- unwound from feed bobbin 52, and is continuously passed in-the direction of .its length-through thetip of the thermal plasma 40 which is presentin the air atmosphere. The yarn speed andexact portion of the thermal plasma transversed are varied to producethe surface treatment residence times, and maximum yarn temperatures indicated. The position of the yarn withinthe tip of the thermal plasma 40 is governed by the locations of rollers 54 and 56.

The resulting surface'treated yarn 58 is next passed through liquid resin bath 60 consisting of a liquidepoxy resin-hardener mixture at ambient temperature (i.e. 25 C.) by `the aid of immersed roller 62, and theresin lmregnated surface treated yarn is Ataken up on bobbin 64 with the aid of roller 66. The liquid -resin system was provided as a solventless system which contained parts by weight epoxy resin and about 87 parts by weight of anhydride curing agent.` 'l

Composite` articles were next formed employing the resin impregnated surface modified yarn. The composite articles were rectangular bars consisting of about 50 percent by volume of the yarn and having dimensions of l/s inch x ld inch x 5 inches. The composite articles were formed by unidirectional layup `of the required quantity of impregnated yarn in a steell mold and compression molding the layup for2 hours at 95 C., and 3 hours at 200 C. in a heated platen press at about 100 p.s.i. pressure. The mold was cooledslowly and the composite article was removed from the mold cavity and cut to size for testing.

The horizontal interlaminar shearstrengths reported were determined by short beam testing of the graphite fiber reinforced composite according to the procedure of ASTM D2344-65T as modified' for straight bar testing at a 4:1 span to depth ratio.

' EXAMPLE I The graphitic carbonaceous yarn undergoing treatment was derived from acrylonitrile homopolymer yarn in laccordance with procedures described in United States Ser. Nos 749,957, filed Aug, 5, i968 and 777,957 filed Nov. 20, 1968. The yarn consisted of a 1600 fil. bundle having a total denier of about 1000. had a carbon content in excess of 99 percent by weight, exhibited a predomininantly graphitic X-ray diffraction pattern, a single filament tenacity of about i3 grams per denier and aV single filament Youngs modulus of about 50 million p.s.i. The following data summarizes the surface treatment conditions employed and the composite properties achieved.

For comparative purposes a composite article was formed as heretofore described employing an identical graphitized carbonaceous yarn without subjecting the same to any form of surface modification` The average horizontal linterlaminar shear strength of the composite article was only 3000 p.s.i.

v EXAMPLE II The graphitic carbonaceous yarn undergoing treatment was derived from a cellulosic precursor and was commercially available from the Union Carbide Corporation For comparative purposes a composite article was formed as heretofore described employing anv identical graphitized carbonaceous yarn without subjecting the same under the designation Thornell 5.0 carbon yarn. The yam consisted of a 720 fil. bundle having a total denier of about 350, had a carbon content in excess of 99 percent by weight, exhibited a predominantly graphitic X-ray diffraction pattern, a `single filament tenacity of about 338,000 p.s.i., and a single filament Youngs modulus of about 52 million p.s.i.

The following data summarizes the surface treatment conditions employed and the composite properties achieved For comparative purposes a composite article was formed as heretofore described employing an identical graphitized carbonaceous yarn without subjecting the same to any form of surface modification. The average horizontal interlaminar shear strength of the composite article was only 3480 p.s.i.

EXAMPLE Il] The graphitic carbonaccous yarn undergoing treatment was derived from acrylonitrile homopolymer yarn in accordance with procedures described in United States Ser. Nos. 749,957, filed Aug. 5, 1968, and 777,957, filed Nov. 20, 1968. The yarn consisted of a 400 fil. bundle having a total denierof about 400, had a carbon content in ex cess of 99 percent by weight, exhibited a predominantly graphitic X-ray diffraction pattern, a single filament tenacity of about 13 grams per denier and a single filament Youngs modulus of about 70 million p.s.i. v.

Th'c following data summarizes the surface treatment conditions employed and the composite properties achieved.

Sample distance Residence Interlaminar from Maximum time ln tip shear enclosed yarn of thermal strength of zone temperaplasma composite Sample (inches) ture C.) (secs.) (p.s.i.)

l 1, 750 3. 2 5, 755 1 l. 750 2. 8 6. 187 l I. 750 2. 3 6. 435 1 1, 750 2. 0 6. 413 1 l, 750 l. 5 6, Ol() to any form ofsurface modification. The average horizontal interlaminar shear strength of the composite article was only 2867 p.s.i.

In each of the aboveexamples the single filament tensile properties of the graphitized carbonac'eous yarns undergoing surface modification were substantially unimpaired as a result of the surface treatment.

Although the invention has been described with preferred embodiments, it is to be understood that variations and modifications may be resorted to as will be apparent to those skilled in the art. Such variationsV and modifications are to be considered within the purview and the claims appended hereto.

l claim:

1. A process for the surface treatment of a predominantly graphitic carbonaceous fibrous material comprising: y

(a) electrically inducing heating of a stream of inert gas present within a laterally enclosed zone to a thermal plasma state of at least about one atmosphere pressure -wherein from about 5 to about S0 percent of said gas isin an ionized state, and wherein-the tip of said thermal plasma exhibits a temperature of about 4,000 to 12,000' K. and extends beyond the confines of said laterally enclosed zone into an open atmosphere, and y (b) continuously passing in the direction of its length a continuous length of a previously graphitizedcarbonaceous fibrous material containing at least about 95 percent carbon by weight and exhibiting a predominantly graphitic X-ray diffraction pattern transversely through said top of said resulting thermal plasma present in said open atmosphere .for a resi-A dence time of about 0.4 to 3.5 seconds wherein said previously graphitized carbonaceous fibrous material is heated to a maximum temperature of about 1500 to 2200 C. and the surface characteristics thereof are beneficially modified and rendered capable of enhanced adhesion with a matrix material.

2. A process according to claim 1 wherein said continuous length of previously graphitized carbonaceous fibrous material contains at least about 99 percent carbon by weight and exhibits a predominantly graphitic X-'ray diffraction pattern.

3. A process according to claim 1 wherein said conf tinuous length of 'previously' graphitized carbonaceous fibrous material is one or more continuous multifilament yam.

4. A process according to claim 1 wherein said'stre'am of inert gas is selected from the gen.' argon, and helium.

5. A process according to claim 1 wherein said previously graphitized carbonaceous fibrous material is heated to a maximum temperature of about 1,600 to 2,000 C.

6. A process for the surface'treatment of a predominantly graphitic carbonaceous fibrous material comprismg:

(a) continuously introducing an inertv gas tangentially to the inner peripheral regions of a laterally enclosed zone,

(b) continuously introducing an inert gas longitudinally along the central region of said laterally enclosed zone, with said inert gas b'eing supplied within said laterally enclosed zone at a rate sufficient to establish a pressure of said gas of at least one atmosphere within said zone,

(c) continuously conducting about the outer peripheral boundaries of said laterally enclosed zone a radio frequency current sufiicient to heat said inert gas to the ionization temperature thereof, thereby maintaining said inert gas in a thermal plasma state wherein from about5 to about 50 percent of said gas is in group consisting of nitroscope of l 11 an ionized state, and wherein the tip of said thermal plasma exhibits a temperature of about 4,000 to 12,000" K. and extends beyond the confines of said laterally enclosed zone into an open atmosphere, and

(d) continuously passing in the direction of its length a continuous length of a previously graphitized carbonaceous fibrous material containing at least about 95 percent carbon by Weight and exhibiting a predominantly graphitic X-ray diffraction pattern transversely through said tip of said resulting thermal plasma present in said open atmosphere for a residence time of about 0.4 to 3.5 seconds wherein said previously graphitized carbonaceous fibrous material is heated to a maximum temperature of about 1500 to 2200 C. and the surface characteristics thereof are beneficially modified and rendered capable of euhanced adhesion with a matrix material.

7. A process according to claim 6 wherein said continuous length of previously graphitized carbonaceous fibrous material contains at least about 99 percent carbon by weight and exhibits va predominantly graphitic X-ray diffraction pattern.

8. A process according to claim 6 wherein said continuous length of previously graphitized carbonaceous fibrous material is one or more continuous multifilament yarn.

9. A process according to claim 6 wherein said inert gas is selected from the group consisting of nitrogen, argon, and helium.

10. A process according to claim 6 wherein said previously graphitized carbonaceous fibrous material s'passed transversely through said tip of said resulting thermal plasma present in said open atmosphere for a residence time of about 1 -to 2 seconds wherein said previously graphilized carbonaceous fibrous material is heated to a maximum temperature of about 1600 t0 2000 C.

I References Cited A UNITED STATES PATENTS 10302611. EDWARD J. Martos, Primary Examine-i UQs. ci. xn. 

